As the world moves further into the digital era, the need for integrating data passing capabilities has become almost as important than voice itself. In response to this, Asymmetric Digital Subscriber Line (ADSL) platforms were created, which employ the existing infrastructure of the Plain Old Telephone System (POTS) as paths for multimedia and high speed data communications. While the conventional telephone voice circuit in the POTS system typical utilizes only about 4 KHz bandwidth, the physical wire connection bandwidth is far greater than this.
While the addition of ADSL technology to the POTS system is a monumental achievement, it is not without issue. The POTS system was originally created to transport analog voice (only), and, to many people's point of view, fell victim to the unfortunate feature of being incredibly reliable. This created an ever-increasing compatibility problem, as newer and newer systems had to be backward compatible with older and older consumer premise equipment (CPE). ADSL brings about new challenges to this environment, as it operates in frequency ranges which the POTS system, while supportive of, was never originally designed to accommodate.
To fully understand this, a very basic understanding of ADSL is needed. ADSL technology works on the principle of Discrete Multiple Tones (DMT), where a tone is simply a frequency. That is, data is transported over ADSL by modulating signals on numerous, but discrete frequencies. In the case of ADSL, the frequencies are spaced evenly at 4.3125 KHz intervals, typically ranging from DC to 1.104 MHz (e.g., with 256 discrete frequencies). As stated above, ADSL runs over the POTS system, which can introduce many problems, one of which is interference between the conventional POTS system and ADSL. Because, the POTS system originally was defined to “own” the 0–8 KHz band, to maintain compatibility with the POTS system, ADSL standards have restricted the ADSL spectrum to start at a range above 8 kHz. Typically, most vendors have opted to start ADSL processing at around 30 kHz (e.g., Tone 7 or 8), which is above the human audible range and outside the POTS spectrum. This may vary slightly from one ADSL vendor or implementation to the next; however, the issue of POTS interference must be dealt with regardless of where the ADSL frequency range begins.
While providing separation of spectrums for POTS and ADSL, their co-existence is still complicated by one underlying issue: when POTS was originally defined, it only considered the existence of telephones. Consequently, specifications and standards for compatible equipment were very strict from 0–8 KHz, but almost non-existent beyond that range. Thus, POTS equipment can, and frequently does, produce detrimental interference to ADSL systems running on them. The result of such interference is normally reduced performance, which can include lower connection rates, data error conditions, and, in some circumstances, even data loss or modem disconnection. All of these are challenges the ADSL system designer faces.
To address these issues, ADSL designers and developers went back to the fundamentals. Obviously, the inclusion of appropriately placed high-pass and low-pass filters of significant performance is a very good way to ensure to proper operation of each system operating together. In the ADSL/POTS world, such filter networks are referred to as splitters. At consumer premises, splitters are 3-ported entities, wherein one port goes to the phone line, a second to the phone, and a third to the ADSL modem. A splitter thus is designed to pass high frequencies (e.g., those greater than 8 KHz) with little/no attenuation between the line and ADSL port, and to pass low frequencies (8 KHz and less) with little/no attenuation between the line and phone ports. In theory, inclusion of splitters at both the phone central office (CO) and consumer premise should allow non-interfering operation of POTS and ADSL. However, cost becomes a practical consideration and, hence compromises have been made. Due to such considerations, the quality of filters often is compromised and, while improvements are seen, some POTS interference still gets through to ADSL. A primary source of POTS interference is POTS ringing.
POTS ringing can be exceptionally detrimental to ADSL because it involves such large voltages. In the United States for example, ringing can introduce a 100V peak-to-peak sine wave onto the telephone line. With even a light load on the line, this can result in some leakage current into devices connected to the line, and thus interference tends to occur. Splitters, being a load on the line, have made improvements to reducing interference, but improvements in reducing such interference are still desired.
In an effort to reduce noise and interference associated with DSL, DSL developers and designer have focused efforts on reducing DSL susceptibility to POTS interference algorithmically, in addition to efforts in improved splitter design. For example, Reed Solomon correction, and interleaving as methods have been developed to improve burst error protection. Reed Solomon provides additional parity bytes to an encoded data stream, such that errors detected at the receiver can be corrected, and the stream reconstructed without retransmission. Such corrective behavior is often referred to as Forward Error Correction (FEC). FEC has limitations, as only a maximum number of errors per data symbol can be corrected.
To aid in this, interleaving is an additional benefit when used with FEC algorithms. Interleaving involves temporally spacing consecutive data samples across larger (than sampling) periods of time, such that impulse noise, which affects several samples, does not corrupt adjacent samples of a particular datum (called a DMT symbol). In this manner, errors caused by impulse noise are ‘spread out’ over multiple symbols, thus increasing the likelihood that FEC algorithms will be able to correct errors in each symbol independently. Interleaving can be highly effective in increasing burst immunity, however, comes at a cost, namely, memory. As a consequence, most ADSL implementations have a fairly small amount of interleaving capability. It is additionally noteworthy that the ADSL standard (T1.413) actually provides two modes of operation-interleave (at various depths/latencies) and a ‘fastpath’, which specifies no interleaving. In the fastpath mode, there is no interleaving protection, and thus it is highly susceptible to the aforementioned POTS interference.
Conventional approaches, including FEC and interleaving algorithms, are highly innovative, yet still are subject to bit error conditions caused in ADSL by POTS interference. As mentioned above, POTS ringing can cause severe interference to ADSL systems, due to both high voltages as well as to the duration of impulses.